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CE632Foundation Analysis and Design
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Design
Pile FoundationsPile Foundations
Foundation Analysis and Design: Dr. Amit Prashant
Indian Standards on PilesIndian Standards on PilesIS 2911 : Part 1 : Sec 1 : 1979 Driven cast insitu concrete pilesIS 2911 : Part 1 : Sec 2 : 1979 Bored castinsitu pilesIS 2911 : Part 1 : Sec 3 : 1979 Driven precast concrete pilesIS 2911 : Part 1 : Sec 4 : 1984 Bored precast concrete pilesIS 2911 : Part 2 : 1980 Timber pilesIS 2911 : Part 3 : 1980 Under reamed pilesIS 2911 : Part 4 : 1985 Load test on piles
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IS 2911 : Part 4 : 1985 Load test on pilesIS 5121 : 1969 Safety code for piling and other deep foundations IS 6426 : 1972 Specification for pile driving hammerIS 6427 : 1972 Glossary of Terms Relating to Pile Driving EquipmentIS 6428 : 1972 Specification for pile frameIS 9716 : 1981 Guide for lateral dynamic load test on pilesIS 14362 : 1996 Pile boring equipment  General requirementsIS 14593 : 1998 Bored castinsitu piles founded on rocks  GuidelinesIS 14893 : 2001 NonDestructive Integrity Testing of Piles (NDT) Guidelines
Foundation Analysis and Design: Dr. Amit Prashant
When is it neededWhen is it neededTop layers of soil are highly compressible for it to support structural loads through shallow foundations.Rock level is shallow enough for end bearing pile foundations provide a more economical design.Lateral forces are relatively prominent.I f i d ll ibl il t th it
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In presence of expansive and collapsible soils at the site.Offshore structuresStrong uplift forces on shallow foundations due to shallow water table can be partly transmitted to Piles.For structures near flowing water (Bridge abutments, etc.) to avoid the problems due to erosion.
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Foundation Analysis and Design: Dr. Amit Prashant
Types of PilesTypes of Piles
Steel PilesPipe pilesRolled steel Hsection piles
Concrete Piles
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Precast PilesCastinsitu PilesBoredinsitu piles
Timber Piles
Composite Piles
Foundation Analysis and Design: Dr. Amit Prashant
Steel Piles: FactsSteel Piles: Facts
Usual length: 15 m – 60 mUsual Load: 300 kN – 1200 kNAdvantage:
Relatively less hassle during installation and easy to achieve cutoff level.Hi h d i i f b d f f t i t ll ti
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High driving force may be used for fast installationGood to penetrate hard strataLoad carrying capacity is high
Disadvantage:Relatively expensiveNoise pollution during installationCorrosionBend in piles while driving
Foundation Analysis and Design: Dr. Amit Prashant
Concrete Piles: FactsConcrete Piles: FactsPrecast Piles:
Usual length: 10 m – 45 mUsual Load: 7500 kN – 8500 kN
Castinsitu Piles:Usual length: 5 m – 15 mUsual Load: 200 kN – 500 kN
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Advantage:Relatively cheapIt can be easily combined with concrete superstructureCorrosion resistantIt can bear hard driving
Disadvantage:Difficult to transportDifficult to achieve desired cutoff
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Foundation Analysis and Design: Dr. Amit Prashant
Types of Piles Based on Their Function and Effect Types of Piles Based on Their Function and Effect of Installationof Installation
Piles based on their functionEnd Bearing PilesFriction PilesCompaction PilesAnchor Piles
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Anchor PilesUplift Piles
Effect of InstallationDisplacement PilesNondisplacement Piles
Foundation Analysis and Design: Dr. Amit Prashant
Displacement PilesDisplacement PilesIn loose cohesionless soils
Densifies the soil upto a distance of 3.5 times the pile diameter (3.5D) which increases the soil’s resistance to shearingThe friction angle varies from the pile surface to the limit of compacted soil
In dense cohesionless soilsThe dilatancy effect decreases the friction angle within the zone of influence of displacement pile (3.5D approx.).
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p p ( pp )Displacement piles are not effective in dense sands due to above reason.
In cohesive soilsSoil is remolded near the displacement piles (2.0 D approx.) leading to a decreased value of shearing resistance.Porepressure is generated during installation causing lower effective stress and consequently lower shearing resistance. Excess porepressure dissipates over the time and soil regains its strength.
Example: Driven concrete piles, Timber or Steel piles
Foundation Analysis and Design: Dr. Amit Prashant
NonNondisplacement Pilesdisplacement PilesDue to no displacement during installation, there is no heave in the ground.
Cast insitu piles may be cased or uncased (by removing casing as concreting progresses). They may be provided with reinforcement if economical with their reduced diameter.
Enlarged bottom ends (three times pile diameter) may be provided in cohesive soils leading to much larger point bearing
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provided in cohesive soils leading to much larger point bearingcapacity.
Soil on the sides may soften due to contact with wet concrete or during boring itself. This may lead to loss of its shear strength.
Concreting under water may be challenging and may resulting in waisting or necking of concrete in squeezing ground.
Example: Bored cast insitu or precast piles
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Foundation Analysis and Design: Dr. Amit Prashant
Load Transfer Mechanism of PilesLoad Transfer Mechanism of PilesWith the increasing load on a pile initially the resistance is offered by side friction and when the side resistance is fully mobilized to the shear strength of soil, the rest of load is supported by pile end. At certain load the soil at the pile end fails, usually in punching shear, which is defined as the ultimate load capacity of pile.
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Foundation Analysis and Design: Dr. Amit Prashant
Load Transfer Mechanism of PilesLoad Transfer Mechanism of PilesThe frictional resistance per unit area at any depth
Ultimate skin friction resistance of pile
.z
szQq
S z perimeter of pileS
suQ
sQz
z
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Ultimate point load
Ultimate load capacity in compression
Ultimate load capacity in tension
.pu pu pQ q A
u pu suQ Q Q
u suQ Q
bearing capacity of soilpuq bearing area of pilepA
upQ usQ
uQ
sQ
Foundation Analysis and Design: Dr. Amit Prashant
Point Load capacity of Pile: General Bearing Point Load capacity of Pile: General Bearing Capacity approachCapacity approach
Ultimate bearing capacity of soil considering general bearing capacity equation. Shape, inclination, and depth factors are included in bearing capacity factors
* * *0.5pu c qq cN q N DN
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Since pile diameter is relatively small, third term may be dropped out
Hence Pile load capacity
* *pu c qq cN q N
* *. .pu pu p c q pQ q A cN qN A
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Foundation Analysis and Design: Dr. Amit Prashant
Point Load capacity of Pile: Meyerhof’s (1976) Point Load capacity of Pile: Meyerhof’s (1976) MethodMethod
Granular soils:Point bearing capacity of pile increases with depth in sands and reaches its maximum at an embedment ratio L/D = (L/D)cr.Therefore, the point load capacity of pile is
*. . .pu p q p ulQ A q N A q*0.5 tanul a qq P N Atmospheric pressureaP
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(L/D)cr value typically ranges from 15D for loose to medium sand to 20D for dense sands.Correlation of limiting point resistance with SPT value
N“ value shall be taken as an average for a zone ranging from 10D above to 4D below the pile point.
Saturated Clays:*. . 9. .pu c u p u pQ N c A c A
ul a qq p pa
0.4 4ul aLq N P ND
Foundation Analysis and Design: Dr. Amit Prashant
Point Load capacity of Pile: Point Load capacity of Pile: Vesic’sVesic’s (1977) Method(1977) MethodPile point bearing capacity based on the theory of expansion of cavities
* *. . .pu p up p c oQ A q A c N N1 2
3o
oK qMean effective normal stress at pile end
*rrN If
Reduced rigidity index of soil1
rrr
r
III
avg vol strain at pile end
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Reduced rigidity index of soil
rigidity indextan 2 1 tans s
rs
G EIc q c q
* 4 ln 1 13 2c rrN I
Type of soil IrSand 75150Silt 5075Clay 150250
Baldi et al. (1981):
3r
f c
Iq q
1.7r
f c
Iq q
For mechanical cone resistance
For electric cone resistance
Foundation Analysis and Design: Dr. Amit Prashant
Point Load capacity of Pile: Janbu’s (1976) MethodPoint Load capacity of Pile: Janbu’s (1976) Method
* *. .pu p c qQ A c N q N
60 90o o
2* 2 2 tantan 1 tanqN e
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60 90o o
* * 1 cotc qN N
Clay Sand
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Foundation Analysis and Design: Dr. Amit Prashant
Point Load capacity of Pile: Point Load capacity of Pile: Coyle and Costello’s (1981) Coyle and Costello’s (1981) Method for Granular SoilsMethod for Granular Soils
*. .pu p qQ A q N
* is a function of ratioqLND
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L is length of pile below G.L.D
Foundation Analysis and Design: Dr. Amit Prashant
Point Load Capacity of Pile resting on RockPoint Load Capacity of Pile resting on Rock
Goodman (1980): . 1pu p uQ A q N2tan 45 2N
unconfined compression strength of rockuq
effective friction angle of rock
u labqTo consider the influence of distributed fractures in rock
hi h fl d b h i ll
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5u lab
u design
qqwhich are not reflected by the compression tests on small
samples, the compression strength for design is taken as
Foundation Analysis and Design: Dr. Amit Prashant
Frictional Resistance of Pile: In SandFrictional Resistance of Pile: In SandThe frictional resistance of pile may be computed as
The unit frictional resistance increases with the depth and reaches its maximum at the depth of approximately 15D to 20D, as shown in the adjacent figure.
. .su szQ S L fL
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SoilPile interface friction angle varies from 0.5 ' to 0.8 ‘.Earth pressure coefficient depends on both soil type and pile installation.
vK. . tansz v sLf K f
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Foundation Analysis and Design: Dr. Amit Prashant
Frictional Resistance of Frictional Resistance of Pile: In SandPile: In Sand
Bhushan (1982) suggested that the value of K and K.tan for large displacement piles can be computed as
0.50 0.008 rK D
t 0 18 0 0065K D
Coyle and Castello(1981)
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. tan 0.18 0.0065 rK D
Coyle and Castello (1981) proposed that ultimate skin frictional resistance of pile can be computed as
. .
. . tan . .
su s av
v
Q f S L
K S L
Avg effective overburden
Foundation Analysis and Design: Dr. Amit Prashant
Frictional Resistance of Pile: In SandFrictional Resistance of Pile: In SandZeitlen and Paikowski (1982) suggested that limiting fs is automatically accounted for by the decrease in ’ with effective confining pressure which may be used to compute K and .
5.5log vo
o
Effective vertical stress at the depth of interest
Effective confining stress during triaxial test
FailureEnvelope
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o g g
Friction angle obtained through triaxial testing at some confining pressure .o
Typical values of K from a number of pile tests:
Foundation Analysis and Design: Dr. Amit Prashant
Frictional Resistance of Pile In Clays: Frictional Resistance of Pile In Clays: methodmethod
.s uf cProposed by Tomlinson (1971):
Empirical adhesion factor
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Foundation Analysis and Design: Dr. Amit Prashant
Frictional Resistance Frictional Resistance of Pile In Clays: of Pile In Clays:
methodmethodRandolph and Murphy (1985)Randolph and Murphy (1985):
. . .su uQ c S L
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Sladen (1992):
. . tanhs uf c
,h vo NCKandcorrection factor for soil disturbance on sides
With the above relationships, can be determined as a function of effective overburden and undrained shear strength
1.n
v uC c C1 and n are constants depending on soil properties and type of pile installation
Foundation Analysis and Design: Dr. Amit Prashant
Frictional Resistance of Frictional Resistance of Pile In Clays: Pile In Clays: methodmethod
Proposed by Vijayvergiya and Focht (1972):
2vs uavf c
Mean undrained shear strength
varies with the length of embedded pile
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. .su s avQ f S L
Ultimate skin friction resistance of pile
Value of and are computed as weighted average over the embedded depth of pile
v uc
This method usually overpredicts the capacity of piles with embedded length less than 15 m.
Foundation Analysis and Design: Dr. Amit Prashant
Frictional Resistance of Pile In Clays: Frictional Resistance of Pile In Clays: methodmethodIn saturated clays displacement piles induce excess pore pressure near pile surface during installation which eventually dissipates within a month or so. Hence, the frictional resistance of pile may be estimated on the basis of effective stress parameters of clay in a remolded state.
. tan .s v R vf K
Effective friction angle of remolded clay at certain depth
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1 sin RK
1 sin OCRRK
Effective friction angle of remolded clay at certain depth
Earth pressure coefficient may be estimated as the earth pressure at rest:
For Normally Consolidated Clay
For Over Consolidated Clay
. .su sQ f S LTotal frictional resistance of pile:
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Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in Pile Load Capacity in CohesionlessCohesionless SoilsSoils
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Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in Cohesionless SoilsPile Load Capacity in Cohesionless Soils
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Foundation Analysis and Design: Dr. Amit Prashant
For BoredPiles
For DrivenPiles
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Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in Cohesionless SoilsPile Load Capacity in Cohesionless Soils
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Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in Cohesionless SoilsPile Load Capacity in Cohesionless SoilsIt seems logical that K value shall be close to the coefficient of earth pressure at rest Ko as described in earlier methods. However, type of
installation has a major impact on how the earth pressure may vary from Ko, as shown in the figure below.
e
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Driv
en C
ircul
ar P
ile
Driv
en C
onic
al P
ile
Bor
ed P
ile
Soil movement
Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in Cohesionless SoilsPile Load Capacity in Cohesionless Soils
IS code recommends Kvalue to be chosen between 1 and 2 for driven piles and 1 and 1.5 for bored piles. However, it is advisable to estimate this value based on the type of construction and fair
estimation of the disturbance to soil around pile. Typical values of ratio between K and Ko are listed below.
30
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Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in Cohesive SoilsPile Load Capacity in Cohesive Soils
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0.5For 1 0.5 , but 1 v vu uc c
0.25For 1 0.5 , but 0.5 and 1 v vu uc c
Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in Cohesive SoilsPile Load Capacity in Cohesive Soils
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Foundation Analysis and Design: Dr. Amit Prashant
Meyerhof’s Formula for Driven Piles based on SPT valueMeyerhof’s Formula for Driven Piles based on SPT value
For L/D > 10
A limiting value of 1000 t/m2 for point bearing and 6 t/m2 is suggested
For Sand:For Sand:
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For NonFor Nonplastic silt and fine sand:plastic silt and fine sand:
For Clays:For Clays:
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Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in NonPile Load Capacity in NonCohesive Cohesive Soils Based on CPT dataSoils Based on CPT data
The ultimate point bearing capacity:
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Foundation Analysis and Design: Dr. Amit Prashant
IS:2911 IS:2911 Pile Load Capacity in NonPile Load Capacity in NonCohesive Cohesive Soils Based on CPT dataSoils Based on CPT data
The ultimate skin friction resistance:
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Correlation of SPT and CPT:
Foundation Analysis and Design: Dr. Amit Prashant
Pile Load Capacity: Other Correlations with Pile Load Capacity: Other Correlations with SPT valueSPT value
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Foundation Analysis and Design: Dr. Amit Prashant
D
Point Load Capacity of Pile: Correlation with CPT Point Load Capacity of Pile: Correlation with CPT data by LCPC Methoddata by LCPC Method .pu c beq
q q kEquivalent avg. cone resistance
Empirical bearing capacity factor
Get the average qc valuefor a zone 1.5D above to 1.5D below the pile tip.
Eliminate the qc valuesthat are higher than 1.3(qc)avg or lower than 0 7(q )
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0.7(qc)avg.
Compute the (qc )eq as an average of the remaining qc values.
Briaud and Miran (1991):
0.6 for clay and siltbk
0.375 for sand and gravelbk
Foundation Analysis and Design: Dr. Amit Prashant
Pile Load Capacity: Pile Load Capacity: Correlation with CPT by Correlation with CPT by Dutch MethodDutch Method
D
Compute the average qc value for a zone yD below the pile tip for y varying from 0.7 to 4. Define qc1as the minimum value of above (qc)avg.Average the value of qc for a zone of 8D above the pile tip, and get
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p p, gqc2. Ignore sharp peaks during averaging.Calculate
1 2 150.2
c cp b a
q qq k p
Atmospheric Pressure
1.0 for OCR = 1bk0.67 for OCR = 2 to 4bk
Foundation Analysis and Design: Dr. Amit Prashant
Pile Load Capacity: Correlation with CPT by Dutch Pile Load Capacity: Correlation with CPT by Dutch MethodMethod
1 21 2 150.
2c c
p b a
q qq R R k p
1 f l t i l t tR1 Reduction factor as function of uR c
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2 1 for electrical cone penetrometerR
2 0.6 for mechanicsl cone penetrometerR
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Foundation Analysis and Design: Dr. Amit Prashant
Pile Load Pile Load Capacity: Capacity: CorrelationCorrelationwith CPT data with CPT data in Sand by in Sand by Dutch MethodDutch Method
Electric Cone
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Frictional cone resistance
Mechanical Cone
Foundation Analysis and Design: Dr. Amit Prashant
Pile Load Capacity: Correlation with CPT data in Pile Load Capacity: Correlation with CPT data in Clays by Dutch MethodClays by Dutch Method
Frictional cone resistance
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Foundation Analysis and Design: Dr. Amit Prashant
Allowable Pile CapacityAllowable Pile Capacity
Factor of Safety shall be used by giving due consideration to the following points
Reliability of soil parameters used for calculationMode of transfer of load to soil
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Mode of transfer of load to soilImportance of structureAllowable total and differential settlement tolerated by structure
Factor of Safety as per IS 2911: